Audio signals, like speech or music, are encoded for example to enable efficient transmission or storage of the audio signals.
Audio encoders and decoders are used to represent audio based signals, such as music and background noise. These types of coders typically do not utilise a speech model for the coding process, rather they use processes for representing all types of audio signals, including speech.
Speech encoders and decoders (codecs) are usually optimised for speech signals, and can operate at either a fixed or variable bit rate.
Speech and audio codecs can also be configured to operate with varying bit rates. At lower bit rates, such a codec may work with speech signals at a coding rate equivalent to a pure speech codec. At higher bit rates, the codec may code any signal including music, background noise and speech, with higher quality and performance.
Speech and audio codecs can use linear predictive coding in order to efficiently represent short term correlations in the signal.
Speech and audio codecs typically realise Linear Predictive Coding by adopting an autoregressive (AR) filter to model the short term correlations in the signal. The coefficients of the filter are known as Linear Predictive (LP) coefficients where the number of coefficients is determined by the order of the AR filter.
The LP coefficients can be determined using linear predictive analysis. The analysis may be regularly performed on the input audio or speech signal for different instances of time, resulting in frequent updates to the set of LP coefficients.
LP coefficients are typically transformed to Line Spectral Frequencies (LSF) in order to facilitate the storage or transmission over a communication channel.
Variable bit rate speech and audio coding can use a varying number of LP coefficients when coding the input speech signal.
Furthermore a variable bit rate speech and audio codec also may support different bandwidths with the same number of LP coefficients.
For example, a variable bit rate speech and audio codec may vary the LPC coding order depending on the bandwidth or sampling frequency of the signal to be coded.
Alternatively, a variable bit rate speech and audio codec may code an audio signal sampled at any one of a number of sampling rates using the same or similar LPC coding order. Consequently any change to the LP coding order or sampling frequency is required to be signalled to the decoder, as this can result in a change in the mode of decoding.
Variable bit rate codecs can transmit the LP coding order or sampling rate in the form of an index flag as part of the coded bit stream.
However, transmission of the LP coding order or sampling rate incurs the penalty of additional bits in the encoded bit stream.